Method and device for atomic interferometry nanolithography

ABSTRACT

The invention proposes a novel technique for implementing high performance atomic lithography, and in particular high resolution lithography. The technique makes use of Stern-Gerlach type atomic interferometry enabling disturbances to be implemented in the atomic phase of the beam. Such interaction then directly modulates the intensity of the associated wave in the plane extending transversely to the beam of atoms, and does so in controllable manner. The installation of the invention for nanolithography by atomic interferometry comprises a Stern-Gerlach type interferometer comprising, as its phase object, four-pole magnetic induction having a transverse gradient created by four parallel bars carrying alternating direct currents, bracketed between two separator plates, preceded and followed respectively by a spin polarizer and by an analyzer operating by laser pumping. An additional uniform field is being created by another four additional bars powered in paired manner by adjustable currents in order to create a uniform field of arbitrary intensity and orientation for the interference pattern by adjusting the two current parameters. The source of atoms is a source that continuously discharges metastable helium or argon with approximately Maxwell type speed dispersion of about 30% to 40% in order to obtain a central spot.

The invention relates to a method of performing lithography at nanometerscale by using atomic interferometry, and it also relates to aninstallation for implementing such a method.

BACKGROUND OF THE INVENTION

The invention relates to the field of lithography at sub-micron scale.By projecting beams of atoms either of alkali type (e.g. Na, Li, or Cr)or of metastable type (He* or Ar*), atomic lithography makes itpossible, via a mask, respectively to cause substance to be deposited ona substrate to be treated or to cause a pattern to be etched in a resindeposited on that substrate. The article by M. Kreis et al., publishedin Applied Physics, Vol. B63, 649 (1996), illustrates that type oftechnique.

Compared with the more conventional technique of photon lithography,atomic lithography presents advantages which relate to theimplementation conditions and to the physical limits of thesetechniques:

-   -   the photon source, generally a UV laser, requires high        brightness and means that are complex and expensive for        producing photons at shorter and shorter wavelengths in order to        increase the resolution of the installation (e.g. an Nd:YAG type        laser for exciting a supersonic jet of xenon atoms);    -   a magnifying optical system (magnifying by a factor of 4 or 5)        formed by multilayer mirrors of reflectivity that is selective        in wavelength and of limited lifetime; and    -   pattern thickness is limited by the wavelength used, e.g. 157        nanometers (nm) for the above-mentioned xenon jet in devices for        producing extreme UV radiation close to soft X-rays.

Micro-lithographic techniques based on atomic optics use thermal orquasi-supersonic beams of atoms that are confined in a magneto-opticaltrap. In such techniques, the beam of atoms is collimated by lasercooling and it interacts with an optical mask. A mask of this type isgenerally formed by a standing lightwave that is blue-shifted relativeto the atomic transition frequency, thus creating a periodic repulsivepotential on the path of the atoms. Such a potential acts as a gratinghaving a pitch equal to half the optical wavelength. Such applicationsare described, for example, in the article by E. M. Rasel published inPhysical Review Letter, Vol. 75, 2633 (1995).

It is thus possible to deposit or etch a series of parallel lines on thesubstrate, or by using two crossed masks it is possible to obtain aperiodic array of predetermined geometrical shape (square, rectangular,or lozenge-shaped).

The atomic technique is, by its very essence, not limited by thewavelength of the associated wave since it is of angstrom order, unlikethe above-described optical methods. Nevertheless, atomicmicro-lithography used in interaction with optical masks requiresmasking potential at a high level of intensity, and thus requires thelightwave that creates the potential to be of high intensity, in orderto obtain significant disturbance of the trajectories of the atoms.Thus, such an installation is not flexible in use. In addition, theresolution that is obtained is limited by the performance of the maskused.

OBJECTS AND SUMMARY OF THE INVENTION

The invention proposes a novel technique of implementing atomiclithography, while avoiding the above-mentioned drawbacks. Thistechnique uses Stern-Gerlach type atomic interferometry under specialconditions that make it possible to achieve atomic phase disturbances ofthe beam rather than trajectory disturbances. Such interactionmodulates, both directly and in controllable manner, the intensity ofthe associated wave in the plane extending transversely to the beam ofatoms.

More precisely, the invention provides a method of lithography by atomicinterferometry on a target, on the basis: of spin polarizing a beam ofincident atoms by optical pumping; of passing through a phase object bytransverse magnetic induction acting on a coherent superposition ofstates; and then of analyzing the beam of atoms by optical interactionso as to retain only a single spin state, the emerging beam of atomscontaining a series of interference terms; wherein the magneticinduction presents a transverse gradient to form an annular interferencepattern, wherein the beam of atoms presents a speed distribution greaterthan 20% in order substantially to eliminate interference fringes otherthan the central fringe which then forms a spot, and wherein adjustabletransverse uniform magnetic induction is added to the induction having agradient so as to move the central spot in translation in predeterminedmanner over the target.

The use of a broad distribution of atom speeds, of the Maxwell type,causes contrast to drop off quickly with interference order, i.e. itreduces the contrast of the rings of the interference pattern outsidethe central spot.

In a particular implementation, the gradient of the transverse inductionis adjusted as a function of the desired spot intensity and diameter.The resulting spot becomes finer and more intense with increasinggradient. It is the limit value of resolution in three dimensions of theapparatus used that finally determines the magnitude of the fieldgradient to be applied.

The invention also provides an installation for performingnanolithography by atomic interferometry implementing the above method.

In one embodiment, the installation comprises an installation fornanolithography by atomic interferometry, the installation comprising aStern-Gerlach type interferometer with a phase object in the form offour-pole magnetic induction with a transverse gradient created by fourparallel bars carrying alternating direct currents, bracketed betweentwo separator zones, preceded and followed respectively by a spinpolarizer and by an analyzer operating by laser pumping, the additionaluniform field being created by four other bars of the same length as andplaced at 45° to the preceding bars and carrying paired currents.

The internal energy of the atoms is quite sufficient for etching a filmof polymer resin placed on a substrate, for example their internalenergy can be greater than or equal to 15 electron volts (eV), and thebeam intensity can be a few 10⁹ atoms per second.

According to preferred characteristics:

-   -   the source of atoms is a source for continuously discharging        metastable helium or argon with an approximately Maxwell type        speed dispersion of about 30% to 40% around approximately 2        kilometers per second (km/s) for atoms of helium and 500 meters        per second (m/s) for atoms of argon;    -   the laser pumping of the polarizer is performed by a circularly        polarized laser diode, the spin polarization being performed on        Zeeman level +J or −J;    -   the analysis is performed by deflecting atoms that lie in Zeeman        states other than the selected state by using different light        frequencies by means of at least one acousto-optical modulator        coupled to the laser in the presence of a magnetic field that is        intense; and    -   the separator plates are made up of physically-implemented        gratings with ultrafine slits, or of optical gratings formed by        a standing wave produced by laser radiation reflected on a        mirror, or else by means of a very low intensity magnetic field        turning through 90° to induce transitions between the Zeeman        states.

In a preferred embodiment, the beam of atoms is collimated by transversecooling by means of a two-dimensional optical molasses made up of twosuccessive identical molasses acting respectively on one or the other ofthe transverse components, using laser beams that are broadened infrequency, being red shifted by means of an acousto-optical modulatorand circularly polarized, and each molasses is formed by a series of“zigzag” reflections of the laser beam on two facing plane mirrors. Thetransverse speeds are then highly limited, being of the order of 0.1 m/sor less.

A constant magnetic gradient is of use only in the vicinity of the axisof a beam of approximately the same size as the pattern to be etched,e.g. a few microns. In particular embodiments, the gradient of thetransverse magnetic field may be created by two coils in an“anti-Helmholtz” configuration, or by a set of electromagnets placed ina multipolar or 2n-polar configuration. These means enable thetransverse field configuration to be adapted to the desired phaseportrait for achieving a predetermined interference pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from thefollowing detailed description relating to embodiments given asnon-limiting examples with reference to the accompanying figures inwhich:

FIG. 1 is an overall diagram of a nanolithography installation of theinvention;

FIG. 2 is a simplified perspective view of an interference patternobtained in the plane extending transversely to the beam of atoms whenit is in the form of a monokinetic beam;

FIG. 3 is a similar perspective view when the beam of atoms has aMaxwell type speed distribution of 40%;

FIG. 4 is a perspective view of an example of how the interferencepattern can be moved in translation by adding a uniform magnetic field;

FIGS. 5 a to 5 d are interference profiles of an installation of theinvention as obtained under various operating conditions; and

FIG. 6 shows examples of interference background around the central spotbeing reduced by using multiple interferometers.

MORE DETAILED DESCRIPTION

The nanolithography installation shown in FIG. 1 is based on aStern-Gerlach type interferometer. The component elements of such aninterferometer comprise in succession, along a central axis Z′-Z, apolarizer 3, a set of bars 51 producing a transverse magnetic fieldforming the phase object between two separator plates 4 and 6, followedby analyzer 7 and then a detector 8 which is located at the position onthe substrate where deposition or etching is to take place.

The atomic interferometer receives a beam of metastable helium atoms He*delivered by a source 1. The source has a continuous electricaldischarge triggered in expansion through a nozzle between a conicaltungsten electrode and the nozzle. The discharge takes place at avoltage of about 1 kiloelectron volts (keV) and at a current of severalmilliamps (mA). Advantageously, the discharge can be pulsed and theresulting gas can be cooled, e.g. using liquid nitrogen.

The density at which atoms are delivered is then greater than about4×10¹⁴ atoms per second per steradian (atom/s/srd) over an angular rangeof 0.5 radians (rd), and the speed distribution of the atoms is of theMaxwell type, and is broad, being about 30% about a mean value of 2km/s. By superposing interference patterns having different pitches,such dispersion give rise to interference rings with contrast that fallsoff quickly. As shown in FIG. 2, in the absence of any dispersion inspeed, the interference pattern F presents constant contrast betweenbright interference fringes I_(b) and dark interference fringes I_(s).However with speed dispersion of about 30%, associated with Maxwell typedistribution, only the bright central fringe I_(c) forming a finecentral spot remains, as can be seen in FIG. 3, which spot stands outsignificantly from the other rings which are highly attenuated along theZ′-Z axis. FIGS. 2 and 3 are simulations which take account of all ofthe parameters that are quantified in the present description.

The angular aperture of the helium jet output by the source 1 is definedby the collimation performed by transverse cooling. This cooling isimplemented by means of two optical molassess 2 a and 2 b acting on thetransverse speed components of the atoms, respectively along the X′-Xaxis and along the Y′-Y axis. They use two laser beams 2 f withred-shifted broadened frequency side bands of width 15 MHz to 20 MHz,the spectral offset and width being obtained using an acousto-opticalmodulator (not shown). These laser beams are circularly polarized. Thebeams 2 f are subjected to a series of reflections between two facingplane mirrors M located to form a “zigzag” of width equal to 8millimeters (mm), the interaction length then being raised to 8centimeters (cm) for each molasses. The working laser power remains low,about 40 milliwatts (mW). Under such conditions, the beam of atomspresents an aperture of about 0.1 rd and transverse speeds of less than0.1 m/s.

The optical interaction between the beam of helium atoms and thepolarizer 3 selects a Zeeman sublevel relative to the quantization axisof the polarizer (with magnetic field) implemented by light pumping inone embodiment. This pumping is obtained by means of a laser beampolarized by a distributed Bragg reflector (DBR) diode having awavelength of 1.08 micrometers (μm) and power of 1 mW. This laser beamis colinear with a magnetic field having intensity of about one Gauss.

The beam is thus subjected to spin polarization through the polarizer 3.The separator plate 4 transforms the Zeeman state selected by thepolarizer, e.g. the +1 state or the −1 state into a coherentsuperposition of identical states for all of the atoms. Thecharacteristics of the resulting interference pattern are a function ofthe phase object 5 which is constructed by the coherent superpositionsof the Zeeman states obtained using the separator zone 4. zm In thisexample, this separation effect is implemented by means of a lowintensity magnetic field, e.g. of 10 milligauss (mG) turning through 900over a distance of about 8 mm in order to induce “Majorana” typetransitions between the Zeeman states.

In this example, the phase object 5 is constituted by a profile ofmagnetic induction B formed by four parallel copper bars 51 that areabout 10 cm long and that are regularly distributed around the axis Z′-Zat a distance a of about 16 mm in this example. The bars carryalternating direct currents ±i_(A) of a few hundreds of milliamps, thebars being parallel to the axis Z′-Z. The magnetic induction B presentsa transverse gradient that is radial and constant in the vicinity of theaxis Z′-Z, and the field is said to be a “four-pole” field. Theinterference pattern F that is obtained is then annular in thetransverse plane (X′-X, Y′-Y).

The four-pole magnetic field B of transverse gradient formed by the fourparallel bars 51 induces a phase object for each Zeeman state with aphase shift that is proportional to the integral of the magnetic field Balong the path followed by the atom, and that is inversely proportionalto the speed of the atom under consideration. The phase shift isproduced on the external movement relating to each Zeeman state.

The installation makes it possible to obtain a gradient G of about5×10⁻⁴ i_(A)/mm, but greater values, e.g. those used for simulations, ofthe order of 10−² i_(A)/mm can easily be obtained by using otherdevices, e.g. coils in an anti-Helmholtz configuration or electromagnetsin a hexapolar configuration.

The diameter of the central spot becomes finer with increasing gradientG. The installation gives gradients that are relatively modest, but itenables a spot to be obtained of a size that is about 10 nanometers. Thegradient G is kept constant in the vicinity of the axis Z′-Z which is ofa size similar to that of the pattern to be etched, e.g. a fewmicrometers. Only the initial width of the profile of the beam of atomsleaving the source is large relative to the size of the pattern to beetched or to be deposited.

The second separator plate 6 is made and operates in the same manner asthe first separator plate 4 to form coherent superpositions for eachatomic state emerging from the phase object. Other techniques exist forforming such a separation, e.g. diffraction through physically-embodiedgratings having ultrafine slits, e.g. at 20,000 lines per millimeter, oroptical gratings formed by a standing wave produced by laser radiationreflected on a mirror or by the reversal effect obtained by absorbing aresonant photon.

The analyzer 7 retains only a single Zeeman state so that the emergingflux of atoms contains only a series of interference terms formingannular bright and dark fringes of intensity that is measured by thedetector 8. In the embodiment described, the analyzer 7 is constitutedusing a laser beam of the same type as that which forms the polarizer 3.This detector is substituted by the silicon substrate coated in resinwhich is to be etched or on which the desired structure is to bedeposited.

Analysis is performed optically by radiation pressure deflecting theatoms which are in Zeeman states other than the selected state, usingdifferent light frequencies that are adjusted by one or twoacousto-optical modulators operating with the same laser as thepolarizer, in the presence of a magnetic field of the order of 100 gauss(G).

In order to enable patterns of predetermined shape to be etched ordeposited on the substrate to be treated, an additional uniform magneticfield B_(h) is produced by four bars 52 disposed at 45° to the firstbars 51 in the transverse plane (X′-X, Y′-Y), the bars being fed withpaired currents ±i_(H) of adjustable magnitude, e.g. about 0.1 A. Suchan additional field then makes it possible to move the central spot intranslation in the transverse plane of the substrate; the gradient ismerely displaced along the axis X′-X about a new origin. FIG. 4 showsthe central spot being shifted in translation along the axis X′-X.

In practice, it is possible to obtain any shift in translation in thetransverse plane by feeding the bars 52 which are paired in turningorder with adjustable currents +i₁, +i₂, −i₁, −i₂ so as to provide twoparameters (i₁, i₂) and deliver a uniform field of arbitrary orientationand intensity.

It should be observed that if the gradient G of the main magnetic fieldB is increased by increasing i_(A), then the intensities i_(H), i₁,and/or i₂ must also be increased in order to shift the spot by the sameamount, since the magnitude of the shift depends on the ratioi_(A)/i_(H). For this purpose, the bars 52 delivering the additionalfields can be replaced by two pairs of coils in a Helmholtzconfiguration.

Interference pattern profiles corresponding to varying implementationsare shown in FIGS. 5 a to 5 d respectively, which figures were obtainedby simulation.

The corresponding conditions are summarized in the table below, as afunction of: source width (a); of relative width of the longitudinalspeed distribution of the beam (δv/v); of the diameter of the resultingcentral spot (δx); and of the distances of the main components of theinterferometer (d=distance between the source and the phase object,L=the width of the phase object, and D=the distance between thedetector/substrate and the source).

TABLE FIG. σ (μm) δv/v d (cm) L (cm) D (cm) G/μm δx (nm) Sources 5a 0.21 5 5 11 4 × 10⁻⁶ 40 coherent 5b 2000 2 5 20 200 4 × 10⁻⁷ 100 coherent5c 500 1 5 10 25 8 × 10⁻⁷ 80 incoherent Zeeman state M = 0 5d 2000 1 510 100 4 × 10⁻⁷ 120 coherent Zeeman state M = 0

These examples show that spot diameters of one to a few tens ofnanometers can easily be achieved under normal conditions. It should beobserved that departures from the ideal shapes shown in FIGS. 5 a to 5 dgive rise to deformation of the interference pattern but that thisdeformation is of no significance in any event.

It should be observed that the intensity of the emitted atomic current,i.e. the number of atoms per second and per square centimeter is equalto the intensity of the current on the resulting central spot, since themodulation implemented by the gradient acts only on the backgroundintensity which is 2J+1 times smaller, where J is the spin of the atomused. For example, for argon (J=2), the background is five timessmaller, and for a source emitting at 4×10¹⁴ atoms/s/srd a currentdensity of 1.3×10¹⁰ atoms/s.cm² is obtained when the phase objectdistance D is equal to 0.5 m.

It should also be observed that in the etching or deposition mechanism,it is the internal energy of the metastable atoms used which performsthe main function rather than the kinetic energy thereof, since theeffect of speed is negligible.

For argon, the internal energy is about 15 eV per atom transferred tothe resin, for helium the internal energy is greater than or equal to 20eV. This energy is quite sufficient for etching a polymer film.

In the deposition process, the speed of the atoms is limited and theatom reflection factor becomes negligible. This applies to thermalspeeds for “depositable” atoms of the alkali type or chromium.

In any event, it is the number of atoms reaching the target during adetermined time interval which needs to be taken into consideration andnot the speed of the atoms. By way of example, a threshold density canbe about 10¹⁴ atoms/s/srd. In particular, the “depositable” atomspresent a flux that is considerably greater (about 10⁴ times greater)than that of the metastable atoms, leading to a current density of about1014 atoms/s.cm².

For etching, the metastable atoms do not dig directly into the resincovering the silicon medium, but they modify its properties by breakingits molecules with efficiency approaching unity. For example, severaltens of atoms reaching an area having a diameter of 40 nm produce thedesired effect which takes about 60 s. For deposition purposes, in orderto deposit a thickness of 1 nm on the same area, it is necessary todeposit 10.5 d/M_(A) where d is the density and M_(A) is atomic mass,and this requires a duration of a few milliseconds.

The atomic spot appears on a background of uniform intensity which isequal to 1/(2J+1) of the intensity of the central spot. When the tracingspeed is relatively low (about 0.7 nm/s), accumulated backgroundintensity can become a drawback. This disturbance can be avoided byusing a multiple interferometer, using a succession of interferometersplaced in series, the analyzer of one interferometer being used as thepolarizer of the next interferometer. FIG. 6 shows the final transverseprofile as modulated by the successive interferometers when the number nof successive identical interferometers in series varies over the range1 to 4. Contrast is thus very significantly improved since if M(ρ)designates the modulation induced by each interferometer, the finaltransverse profile is M(ρ)^(n). The half-height diameter of the centralspot is also reduced, by a statistical factor equal to √{square rootover (n)}.

The invention is not limited to the embodiments described and shown. Therelationship describing the transverse distribution of density of atomsin the beam can be of the Gaussian type, for example, with a standarddeviation of 100 μm.

1. A method of lithography by atomic interferometry on a target, on thebasis: of spin polarizing a beam of incident atoms by optical pumping;of forming a phase object by transverse magnetic induction on the basisof a coherent superposition of spin states; and then of analyzing thebeam of atoms by optical interaction so as to retain only a single spinstate; the emerging beam of atoms containing a series of interferenceterms; wherein the magnetic induction presents a transverse gradient toform an annular interference pattern, wherein the beam of atoms presentsa speed distribution greater than 20% in order substantially toeliminate interference fringes other than the central fringe which thenforms a spot, and wherein adjustable transverse uniform magneticinduction is added to the induction having a gradient so as to move thecentral spot in translation in predetermined manner over the target. 2.A method of lithography according to claim 1, in which the gradient ofthe transverse induction is adjusted as a function of the desired spotintensity and diameter, the resulting spot becoming finer and moreintense with increasing gradient.
 3. An installation for nanolithographyby atomic interferometry, the installation comprising a Stern-Gerlachtype interferometer with a phase object in the form of four-polemagnetic induction with a transverse gradient created by four parallelbars carrying alternating direct currents, bracketed between twoseparator plates, preceded and followed respectively by a spin polarizerand by an analyzer operating by laser pumping, the additional uniformfield being created by four other additional bars fed in paired mannerwith adjustable currents in order to create a uniform field of arbitraryorientation and intensity by adjusting two parameters.
 4. Aninstallation for nanolithography by atomic interferometry according toclaim 3, in which the additional bars have the same length as the mainbars, are disposed at 45° relative thereto, and carry paired currents.5. An installation for nanolithography according to claim 3, in whichthe additional field bars comprise two pairs of coils in Helmholtzconfiguration.
 6. An installation for nanolithography by atomicinterferometry according to claim 3, in which the source of atoms is asource for continuously discharging metastable helium or argon with anapproximately Maxwell type speed dispersion of about 30% to 40% aroundapproximately 2 km/s for atoms of helium and 500 m/s for atoms of argon.7. An installation for nanolithography by atomic interferometryaccording to claim 3, in which the laser pumping of the polarizer isperformed by a circularly polarized laser diode, the spin polarizationbeing performed on Zeeman level +1 or −1.
 8. An installation fornanolithography by atomic interferometry according to claim 3, in whichthe analysis is performed by deflecting atoms that lie in Zeeman statesother than the selected state by using different light frequencies bymeans of at least one acousto-optical modulator coupled to the laser inthe presence of a magnetic field that is intense, e.g. of the order of100 G.
 9. An installation for nanolithography by atomic interferometryaccording to claim 3, in which the separator plates are made up ofphysically-implemented gratings with ultrafine slits, or of opticalgratings formed by a standing wave produced by laser radiation reflectedon a mirror, or else by means of a very low intensity magnetic fieldturning through 90° to induce transitions between the Zeeman states. 10.An installation for nanolithography by atomic interferometry accordingto claim 3, in which the beam of atoms is collimated by transversecooling by means of a two-dimensional optical molasses made up of twosuccessive identical molasses acting respectively on one or the other ofthe transverse components, using laser beams that are laterally enlargedin frequency, being shifted towards the red by means of anacousto-optical modulator and circularly polarized, and each molasses isformed by a series of “zigzag” reflections of the laser beam on twofacing plane mirrors.
 11. An installation for nanolithography by atomicinterferometry according to claim 3, in which the gradient of thetransverse magnetic field is created by two coils in an “anti-Helmholtz”configuration, or by a set of electromagnets placed in a multipolar or2n-polar configuration.
 12. An installation for nanolithography byatomic interferometry according to claim 3, in which the source makesuse of a discontinuous electrical discharge struck in expansion througha nozzle between a conical electrode made of tungsten and the nozzle,thereby forming jets of atoms either of the alkali or chromium type, orelse of the metastable hydrogen or inert gas type to obtain respectivelydeposition on a substrate that is to be treated or etching of a patternin a resin placed on the substrate.
 13. An installation fornanolithography by atomic interferometry according to claim 3, in whichthe interferometry is multiple in that use is made of a succession ofinterferometers in series, the analyzer of one interferometer being usedas the polarizer of the next in order to eliminate the background fromthe interference pattern and in order to refine the central spot.